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Mosquito-Borne Disease

West Nile virus (WNV) is a mosquito-borne disease first detected in the United States in 1999.  This disease is also found in Africa, Asia, the Middle East, Canada, and Mexico. WNV first appeared in California in 2003 and by 2004, WNV had spread to all 58 counties. Infection by WNV is minimized through the concerted actions of local and state public health and vector control agencies. People can protect themselves by taking precautions to avoid being bitten by mosquitoes.

Mosquito-borne encephalitis/encephalomyelitis refer to viral diseases that are transmitted by mosquitoes. The term encephalitis refers to an inflammation of the brain; encephalomyelitis refers to an inflammation of the brain and spinal cord. In California, West Nile virus (WNV), St. Louis encephalitis (SLE), and western equine encephalomyelitis (WEE) are the three most important viral mosquito-borne viruses that can cause encephalitis/encephalomyelitis. The viruses that cause these diseases are maintained in nature through a mosquito-bird-mosquito cycle. Birds that are common throughout California, such as the American crow, English house sparrow and the house finch, are important carriers of these viruses. Horses are also particularly susceptible to infection with WNV and WEE, but there is a vaccine for horses to prevent these diseases.

Zika Virus is transmitted by Aedes aegypti mosquitoes (also known as yellow fever mosquitoes) and Aedes albopictus mosquitoes (also known as Asian tiger mosquitoes). These mosquitoes are not native to California. However, since 2011 they have been detected in several California counties. An Aedes mosquito can only transmit Zika virus after it bites a person who has this virus in their blood. Thus far in California, Zika virus infections have been documented only in people who were infected while traveling outside the United States or through sexual contact with an infected traveler. To date there has been no local mosquito-borne transmission of Zika virus in California. 

Zika virus is not spread through casual contact, but can be spread by infected men to their sexual partners. There is a growing association between Zika and microcephaly (abnormally small head and brain) in newborns, as well as Zika and Guillain-Barré Syndrome (GBS), a disease affecting the nervous system. Studies are ongoing to further evaluate these associations.

Dengue is a disease caused by any one of four closely related dengue viruses (DENV-1, DENV-2, DENV-3, and DENV-4). People get dengue from the bite of an infected mosquito. The mosquito becomes infected when it bites a person who has dengue virus in their blood. It takes a week or more for the dengue virus to replicate in the mosquito; then the mosquito can transmit the virus to another person when it bites. Dengue is transmitted principally by Aedes aegypti (yellow fever mosquito) and Aedes albopictus (Asian tiger mosquito). These mosquitoes are not native to California, but infestations have been identified in multiple counties in California. Dengue virus cannot be transmitted from person to person.

Malaria is a mosquito-borne disease caused by small, one-celled parasites called Plasmodium that infect and destroy red blood cells. Four different Plasmodium can cause disease in humans: Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, andPlasmodium vivax.

Rift Valley fever (RVF) is an acute, fever-causing viral disease most commonly observed in domesticated animals (such as cattle, buffalo, sheep, goats, and camels), with the ability to infect and cause illness in humans. The disease is caused by RVF virus (RVFV), a member of the genus Phlebovirus in the family Bunyaviridae

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